Drilling tools are used when drilling a well in the subterranean formation. However, drilling may also be performed in an existing well for making a branch well or for reboring a stuck valve. An example of a drilling tool is disclosed in U.S. Pat. No. 3,225,843, which tool is anchored to the formation while drilling in order to transfer as much of the energy available as possible to the drilling process, and thereby gain more drilling power.
The subterranean formation may vary in hardness through the formation. Thus, when drilling in a soft part of the formation, the drilling force need not be as great as when drilling in the hardest part of the formation. While drilling from a soft part of the formation to a harder part of the formation, a drill bit on the drilling head will hit the hard formation which may imply serious damage on the individual cutting edge of the bit.